The species of trees are distributed in well-defined zones, because of which they may to a large degree be segregated, or at least the specific composition of the forest, if a mixed one, may be determined. In altitudinal range, the oaks are found from sea-level to 6,000 feet, within which each species may have its characteristic range. For example, _Quercus suber_ reaches from sea-level to 2,500 feet; _Q. ballota_ from 2,500 to 4,000 feet; and _Q. lusitanica_ from 3,500 to 6,000 feet. _Pinus halepensis_ grows from the sea-level to 3,600 feet, and probably much higher in the Saharan Atlas, while _Cedrus atlantica_ is to be found from 4,000 to 6,000 feet. _Abies barborensis_ is said to attain a higher altitude than the cedar. Among the definitive physical factors by which the composition of a forest is determined—the rainfall, the temperature, the soil, and the altitude—probably of the first rank should be considered the rainfall and the temperature, which are affected by the altitude. In the case of the distribution of the cork oak, however, the character of the soil plays an important rôle. This species grows only on sandy soil, and where the rainfall exceeds 600 mm. Because of the latter requirement the larger part of the cork-oak forested area is east of Algiers, the annual rainfall to the west of that place falling for the most part under 600 mm. Similarly the minor features of distribution, inside of the specific range, may be explained. For example, in the upper limits of its range, _Pinus halepensis_ appears chiefly on southern slopes, as on Mount Gorno, while at the low altitudes it is to be found on the northern face; temperature reactions apparently—familiar phenomena in mountainous districts. Because of the unison of response to common environmental factors, much of the forested area, especially at the higher altitudes, is composed of one species or one species largely predominates. This is true of the pine and the cedar forests to a marked degree; _e.g._, in the pine forests on Mount Gorno and the cedar near Batna.